You need to run the sample in .NET 4.0. Here are the requirements taken from the readme file of the webmail demo:
==== Requirements ====
Telerik RadControls for ASP.NET AJAX Q2 2012 (2012.2.607, full trial or purchased installation)
Visual Studio 2010 (or later)
Microsoft .NET framework 4.0
SQL Server Express Edition 2008 (or later)
==== Connecting to a different instance of SQL Server ====
The application is configured to connect to the (local) instance by default.
If your SQL Server is using a different instance name, update the connection string in the web.config file.
All the best,
the Telerik team
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